Choline chloride and amino acid solutions taste and hydration behavior with experimental thermodynamic properties and COSMO-PC-SAFT calculation

Aqueous amino acid solutions have been introduced as dietary supplements for both animals and humans. This study investigates the physicochemical properties of the solutions containing amino acids (l-glycine, d,l-alanine, l-proline), choline chloride, and water at temperature range of 288.15 to 318.15 K. The results show that increasing concentrations of amino acids and choline chloride lead to higher solution densities. Analysis of apparent molar volume (Vφ) and apparent molar isentropic compressibility (κφ) reveals that Vφ values increase with choline chloride concentration and temperature, indicating enhanced solute–solvent interactions, while κφ values decrease, suggesting increased solution compression. Thermodynamic analysis using the Redlich-Mayer model and COSMO-based modeling provides insights into molecular interactions. However, COSMO-based parameters show high average relative deviation percentage (ARD %) values, indicating poor predictive performance for the density of these systems. In contrast, the ePC-SAFT equation of state effectively predicts the densities, particularly for l-proline-based solutions, which show very low ARD % values, indicating high accuracy. The ePC-SAFT model also performs reasonably well for l-glycine solutions but shows poorer results for d,l-alanine-based solutions. The study also examines the sweetness and saltiness criteria (ASV and ASIC) of these solutions. The ASV values, which serve as a sweetness criterion, are higher than the ideal range of 0.5 < ASV < 0.7, suggesting an overly sweet taste. The ASIC values follow a similar trend, indicating increased saltiness. To achieve an appropriate grade of sweetness and saltiness, dilution to lower concentrations of the solution is recommended. Furthermore, the use of choline chloride is found to increase salt intake and enhance the taste of salt, which can be beneficial in amino acid supplements used in animal food.


Theoretical background
PC-SAFT (perturbed chain SAFT) uses chains of spheres to model fluids.This allows it to handle chain-like molecules and self-association (like hydrogen bonding) through second-order perturbation theory 24,25 .In a canonical ensemble (fixed temperature, volume, particles), the Helmholtz free energy relation provides a convenient way to derive thermodynamic properties for these complex fluids 26 : where, superscripts hc and disp, terms are used for the hard chain and dispersion which are given in the original version of PC-SAFT 13,27 .The symbol assoc was used for association contribution that uses the original SAFT equations 28,29 .The components of the equations have been described in the original paper as presented in these equations 13 .The density could be predicted using the following equation with iteration of pressure 30,31 , where, ρ and Z are the number density and compressibility factor, respectively.The main parameters of this equation of state are segment number, m, segment diameter, σ, dispersion energy, u 0 /K B , association energy, ε AB / K B , and effective association volume, κ AB .These parameters commonly obtained from the experimental vapor pressure or density data at different pressures and temperatures (P,T) 32 .However, there is some indirect methods to obtain these parameters 33 .Recently, a new methodology has been introduced that could be used to predict these parameters with density functional theory results 34 .
Electron density, a crucial molecular property, can be obtained using density functional theory 35 .However, another model, based on quantum mechanics and equilibrium thermodynamics, indirectly predicts various properties via chemical potentials 36 .This model has recently been applied to parameterize a popular equation of state for fluids.Both approaches share a common mathematical framework based on a specific system type (fixed temperature, volume, and particle number) 34 .This allows for a theoretical calculation of the equation of state parameters without needing real-world data.Researchers have exploited this link to derive segment diameters, a key parameter, from the other model's cavity and surface area outputs 37 .
(1) a = a hc + a disp + a assoc (2) a hc = ma hs − i x i (m i − 1) ln(g hs (σ )) (3) where, V stands for the volume of cavity and A represents the corresponding area of cavity, and the a, b, c, and d symbols are the parameters that could be obtained from the correlation of the dispersion energy and association energy with the existing data in literature 34 .The other symbols stand for the PC-SAFT parameters 34 .It should be noted that the association volume has been considered between 0.01 and 0.03 for the studied systems.
The density and sound speed data of choline chloride in aqueous solutions of amino acids (l-glycine, l-proline, d,l-alanine) have been measured at different temperatures (288.15-318.15)K and ambient pressure.The thermodynamic properties were calculated from the experimental data and correlated with Redlich-Mayer model.Also, the densities of the studied mixtures have been predicted with PC-SAFT equation of state variations on the experimentally obtained parameters and COSMO-based obtained parameters.These results were used to determine the taste and hydration behavior and interpretation of the intermolecular interactions.

Materials
All of the amino acids (l-glycine, d, l-alanine, l-proline have been purchased from Merck, and used without more purification while choline chloride was Sigma Aldrich.All of the materials purity were > 99%.Also, deionized ultrapure water with a specific conductance below 1 μS cm −1 was used to prepare the corresponding aqueous solutions of choline chloride in the presence of amino acids.

Apparatus and procedure
An analytical balance (Shimadzu AW-220) with resolution of ±1 × 10 −4 g and accuracy ±2 × 10 −4 g was used to prepare the solution in molal-based concentration.A digital vibrating U-shaped densitometer (Anton Paar DSA5000) with resolution of ± 1 × 10 -6 g cm −3 for density and 0.01 m s −1 for speed of sound while the uncertainty for these properties were 4 × 10 -5 g cm −3 and 0.7 m s −1 , respectively.The instrument has been calibrated with air and distilled water while the frequency for speed of sound measurement was 3 MHz.

Theoretical calculations
A specific computational method (mentioning the general class, GGA, is optional) within a popular software package (Dmol3) was used to optimize molecular geometry.This method has been suggested by the software developers as potentially effective for studying real solvents.The optimization relied on a particular set of calculations (DFT) that provided the necessary data (COSMO results).

Thermodynamic properties
The density (d) and speed of sound (u) data of the aqueous solutions of the amino acids (l-glycine, d,l-alanine, l-proline) and choline chloride were measured under atmospheric pressure (P = 0.086 MPa) at temperature range T = (288.15-318.15)K.These data are given in Table 1.The experimental density data of aqueous amino acid solutions have been compared with literature data as shown in Figs.S1-S3 (supporting information).The data was in good agreement with those reported in the literature data.
Also, the density of the solutions has been increased with addition of the amino acid and choline chloride content and decreased with increasing temperature.The apparent molar volumes and apparent molar isentropic compressibility values of the choline chloride in the amino acid + water solutions were evaluated using the following equation 38 , where, the symbols m, M, d, d 0 , u, κ s , κ s0 are the molality, molar mass of choline chloride, density of the solution, density of solvent, speed of sound, isentropic compressibility of the solution, and isentropic compressibility of  1 by increasing of the concentration of amino acids, the κ s is decreased.Also, the V φ values are plotted in Fig. 1 at different temperatures.
It's clear that when the concentrationof the increased the compression will be harder than before.Also, based on Table 1 the isentropic compressibility is decreased while the temperature is going up.It's obvious by increasing of temperature the density is decreased but the speed of sound is increased too.Based on equation.the κ s and sound velocity has inversion relationship so in total κ s is decreased.The thermodynamic properties of the studied solutions have been studied to achieve a good aspect of view around them.Accordingly, the apparent molar volume and apparent molar isentropic compressibility of the choline chloride have been given with V φ and κ φ .These values are given in Table 2.
The apparent molar properties have been correlated using Redlich-Mayer model as given by following equations: where, V φ 0 , S v , and B v values are given in Table 3, for the binary solutions.The V φ 0 values are a criterion of solute-solvent interaction, while the S v value indicating the solute-solute interactions, and B v is an adjustable parameter.The V φ 0 values of the studied solutions are increased by concentration of choline chloride and at higher temperature in the binary solutions.the κ φ 0 is the partial molar isentropic compressibility and S κ and B κ are the empirical parameters of the equation.The obtained parameters for the investigated solutions are listed in Table 3 for the studied solutions.
The κ φ 0 values increase with the concentration of amino acids.Also, the V φ 0 values temperature dependency are fitted with a second-degree polynomial equation, the empirical parameters of A, B, and C have been used to calculate the standard apparent molar expansibility at constant pressure E φ 0 using following equation 38 (13)  The values of E φ 0 are given in Table 3.The apparent isobaric thermal expansion was evaluated by the following equation 38 , The calculated values of α for choline chloride are given in Table 3.The value of α is a criterion for the apparent molar volume temperature dependency and its response to increment of temperature.The large value of this factor means the system volume is sensitive to temperature change.The pressure would also break the solvent structure and the same reason suggests that the heat capacity decrease.Hepler's 39 determined relation for structure making or breaking behavior of a solute in a solution is given by the following equations, where, ∂ 2 V 0 ϕ /∂T 2 is the constant for the choline chloride are given in Table 3. Negative values of this parameter mean the choline chloride have structure breaker behavior while otherwise it would be structure maker.The taste behavior of the solutions has been investigated using the apparent specific volume (ASV) and apparent specific isentropic compressibility (ASIC) by following relation 40 :  4.
The ASV value have been introduced as sweetness criterion in the literature.Where, acceptable values for this property should be 0.5 < ASV < 0.7 is an ideal range of sweetness while the reported data show higher value rather than this value.Also, the ASIC has been studied that is in agreement with the observed trend as ASV.However, it should be diluted to the lower concentration of the solution to achieve an appropriate grade.Previous studies have been deducted that using choline chloride would be increase salt intake and would enhance taste of the salt 41 .However, the amino acid supplements are usually used for animal food and it has considerable salt ingredient in the mixture.Accordingly, it could be enhancing the amino acid supplementation in the supply chain and improve the quality of the food.
Previous studies have indicated that using choline chloride can increase salt intake and enhance the taste of salt.This is a significant observation, as choline chloride is commonly used as a supplement.In the context of amino acid supplements, which are typically used for animal food, the mixture usually contains a considerable amount of salt.The presence of choline chloride in these supplements could therefore enhance the overall taste due to its salt-enhancing properties.The implications of these findings are noteworthy for the supply chain of amino acid supplements.By incorporating choline chloride, the supplementation process can be enhanced, potentially improving the quality of the food provided to animals.This enhancement comes from the improved taste, which could make the food more palatable and thus more likely to be consumed in adequate amounts by the animals.In summary, the ASV values for the studied solutions are higher than the ideal range, suggesting an overly sweet taste.The ASIC values support this trend, indicating an increased saltiness.Diluting the solutions to lower concentrations is recommended to achieve the appropriate sweetness and saltiness levels.The use of choline chloride not only increases salt intake but also enhances the taste of salt, which can be beneficial in amino acid supplements used in animal food.This can improve the quality and acceptability of the food, ultimately enhancing the supplementation process in the supply chain.

Hydration behavior with COSMO and ePC-SAFT
The σ-profile is essential in COSMO-based thermodynamics, reflecting the charge distribution on a molecule's surface.Acting as a unique fingerprint, it shows the likelihood of specific charge density values in segmented segments.COSMO models, like COSMO-RS and COSMO-SAC, use σ-profiles to predict thermodynamic properties and molecule-environment interactions.These profiles are typically obtained via density functional theory (DFT) calculations, which can be computationally intensive.To mitigate this, alternative methods for approximating σ-profiles are available in software tools and databases, facilitating quicker analysis, especially useful in high-throughput screening applications.
The GGA VWN-BP function in Dmol3 is recommended by the developer and has shown promising results for real solvents.In this study, COSMO results were obtained through DFT calculations using the Dmol3 module of Materials Studio (Biovia, Materials Studio 2023).Molecule geometries were optimized using GGA (VWN-BP).Figure 2 shows the optimized structures and corresponding COSMO results, specifically the σ-profiles, for the solvents and IL under investigation.Table 5 provides the cavity volume, cavity surface area, and the HOMO and LUMO energies of the compounds from Dmol3 energy optimization calculations.The GGA VWN-BP function, a crucial exchange correlation functional used in DFT calculations.It is employed in the Dmol3 module of Materials Studio for electronic structure calculations and simulations.This function combines the exchange functional of Vosko, Wilk, and Nusair with the correlation functional developed by Becke and Perdew.The exchange part facilitates electron exchange between orbitals, while the correlation part accounts for electron interactions.By integrating these components, the VWN-BP function approximates the exchange correlation effects in the system.As a GGA functional, it considers not only the electron density but also its gradient, providing a more accurate representation of the system's electronic structure compared to simpler functionals like the local density Table 2.The apparent molar volume and apparent molar isentropic compressibility of the choline chloride in the ternary systems containing (water + choline chloride + amino acid (l-glycine, d,l-alanine, l-proline) under P = 865 hPa at T = (288.15-318.15)K.The standard uncertainties for molality, temperature and pressure were u (m) = 0.002 mol kg −1 , u (T) = 0.02 K, u (P) = 10 hPa, respectively with level of confidence 0.68.The combined standard uncertainty for apparent molar volume and apparent molar isentropic compressibility were, 10 6 u c (V 0 φ ) = 0.4 m 3 mol −1 , 10 14 u c (κ 0 φ ) = 0.7 m 3 mol −1 Pa −1 with level of confidence 0.95.where the superscripts hc and disp show the hard chain and dispersion force.The symbol assoc was used for association contribution that uses the original SAFT equations.In the case of studied systems, the 2B approach for amino acids and 4C for H 2 O were used.The dipole is represented for the polar contribution (dipole momentum) of the components.The ion term has been calculated based on the PC-SAFT electrolyte extension term.The density could be predicted using the following equation with iteration of pressure, The utilized parameters segment number, m, segment diameter, σ, dispersion energy, ε/K B , association energy, ε AiBi /K B and effective association volume, κ AiBi are given in Table 6.
The given parameters in Table 6 have been used to predict the density of the aqueous solutions of choline chloride in the presence of different concentrations of studied amino acids at different temperatures.The corresponding average relative deviations of these systems are given in Table 7.As could be seen the COSMO-based parameters are not suitable for predictive calculation of the density of the studied systems.However, according to the results the ePC-SAFT model is a successful model in the prediction of density of the studied solutions while the alanine solutions show poor results in the prediction, the proline solutions predicted with high accuracy with experimental data.Accordingly, this equation of state could be used to evaluate the energy of the solvation of these solutions.Also, this equation of state could provide other calorimetric properties of the systems such as enthalpy.These results could be used to interpret the interactions between the existing species in the solutions.
The study compares two models: COSMO-based parameters and the ePC-SAFT model.The COSMO-based parameters are derived from the Conductor-like Screening Model approach, while the ePC-SAFT model is an equation of state tailored for predictive calculations.The amino acids studied include l-glycine (Gly), d,l-alanine (Ala), and l-proline (Pro).The temperatures examined range from 288.15 to 318.15 K, and the concentrations of amino acids in the water + ChCl solutions are 0.05 m, 0.10 m, and 0.15 m.The average relative deviation percent (ARD %) values represent the average relative deviation of the predicted densities from the experimental values, with lower ARD % values indicating better predictive accuracy.For l-glycine solutions, the ePC-SAFT model shows ARD % values around 3.5% to 4.64%, indicating relatively good predictive accuracy.In contrast, the COSMO-based parameters show significantly higher ARD % values (around 11.54% to 12.87%), suggesting poor predictive performance for these systems.For d,l-alanine solutions, the ePC-SAFT model shows ARD % values around 7.75% to 9.45%.These values are higher compared to those for L -glycine solutions, indicating poorer predictive accuracy for Ala solutions.The COSMO-based parameters again show higher ARD % values (around 15.74% to 17.68%), indicating poor predictive performance.The ePC-SAFT model shows exceptionally low ARD % values (ranging from 0.12 to 0.87%) for l-proline solutions, indicating excellent predictive accuracy.The COSMO-based parameters show ARD % values around 8.12% to 8.96%, which are significantly higher than those for the ePC-SAFT model, indicating poor predictive performance.In summary, the COSMO-based parameters are generally not suitable for predicting the density of the studied systems as they consistently show high ARD % values across all amino acids and temperatures.The ePC-SAFT model proves to be successful in predicting the (21) a = a hc + a disp + a assoc + a ion

1 ASV/m 3 kg − 1 ASIC/m 3 kg − 1
www.nature.com/scientificreports/l-Proline has the smallest cavity volume and surface area among the molecules, indicating a more compact solute.It also has the least negative dielectric (hydration) energy, suggesting weaker interactions with the solvent compared to other molecules.The HOMO energies for all molecules are negative, while the LUMO energies vary across the compounds.The modeling of the density of the aqueous solutions of choline chloride and amino acids have been carried out with ePC-SAFT equation of state.The general form of the equation of state could be represented with free Helmholtz energy term by the following relation, m/mol kg −Pa −1

Table 5 .
The results of COSMO calculations including surface area and total volume of cavity, besides the total HOMO and LUMO orbitals number and energy and dielectric (solvation energy) in water.

Table 7 .
The average relative deviation percent of the predicted densities of the aqueous solutions containing choline chloride in the presence of different concentrations of the studied amino acids (l-glycine, d,l-alanine and l-proline) at different temperatures.